1. Field of the Invention
This invention relates to amorphous metal magnetic components; and more particularly, to a generally three-dimensional bulk stamped amorphous metal magnetic component for large electronic devices such as magnetic resonance imaging systems, television and video systems, and electron and ion beam systems.
2. Description of the Prior Art
Magnetic resonance imaging (MRI) has become an important, non-invasive diagnostic tool in modern medicine. An MRI system typically comprises a magnetic field generating device. A number of such field generating devices employ either permanent magnets or electromagnets as a source of magnetomotive force. Frequently the field generating device further comprises a pair of magnetic pole faces defining a gap with the volume to be imaged contained within this gap.
U.S. Pat. No. 4,672,346 teaches a pole face having a solid structure and comprising a plate-like mass formed from a magnetic material such as carbon steel. U.S. Pat. No. 4,818,966 teaches that the magnetic flux generated from the pole pieces of a magnetic field generating device can be concentrated in the gap therebetween by making the peripheral portion of the pole pieces from laminated magnetic plates. U.S. Pat. No. 4,827,235 discloses a pole piece having large saturation magnetization, soft magnetism, and a specific resistance of 20 μΩ-cm or more. Soft magnetic materials including permalloy, silicon steel, amorphous magnetic alloy, ferrite, and magnetic composite material are taught for use therein.
U.S. Pat. No. 5,124,651 teaches a nuclear magnetic resonance scanner with a primary field magnet assembly. The assembly includes ferromagnetic upper and lower pole pieces. Each pole piece comprises a plurality of narrow, elongated ferromagnetic rods aligned with their long axes parallel to the polar direction of the respective pole piece. The rods are preferably made of a magnetically permeable alloy such as 1008 steel, soft iron, or the like. The rods are transversely electrically separated from one another by an electrically non-conductive medium, limiting eddy current generation in the plane of the faces of the poles of the field assembly. U.S. Pat. No. 5,283,544, issued Feb. 1, 1994, to Sakurai et al. discloses a magnetic field generating device used for MRI. The devices include a pair of magnetic pole pieces which may comprise a plurality of block-shaped magnetic pole piece members formed by laminating a plurality of non-oriented silicon steel sheets.
Notwithstanding the advances represented by the above disclosures, there remains a need in the art for improved pole pieces. This is so because these pole pieces are essential for improving the imaging capability and quality of MRI systems.
Although amorphous metals offer superior magnetic performance when compared to non-oriented electrical steels, they have long been considered unsuitable for use in bulk magnetic components such as the tiles of poleface magnets for MRI systems due to certain physical properties of amorphous metal and the corresponding fabricating limitations. For example, amorphous metals are thinner and harder than non-oriented silicon steel. Consequently, conventional cutting and stamping processes cause fabrication tools and dies to wear more rapidly. The resulting increase in the tooling and manufacturing costs makes fabricating bulk amorphous metal magnetic components using such techniques as conventionally practiced commercially impractical. The thinness of amorphous metals also translates into an increased number of laminations in the assembled components, further increasing the total cost of the amorphous metal magnetic component.
Amorphous metal is typically supplied in a thin continuous ribbon having a uniform ribbon width. However, amorphous metal is a very hard material making it very difficult to cut or form easily, and once annealed to achieve peak magnetic properties, it becomes very brittle. This makes it difficult and expensive to use conventional approaches to construct a bulk amorphous metal magnetic component. The brittleness of amorphous metal may also cause concern for the durability of the bulk magnetic component in an application such as an MRI system.
Another problem with bulk amorphous metal magnetic components is that the magnetic permeability of amorphous metal material is reduced when it is subjected to physical stresses. This reduction in permeability may be considerable depending upon the intensity of the stresses on the amorphous metal material. As a bulk amorphous metal magnetic component is subjected to stresses, the efficiency at which the core directs or focuses magnetic flux is reduced. This results in higher magnetic losses, increased heat production, and reduced power. Such stress sensitivity, due to the magnetostrictive nature of the amorphous metal, may be caused by stresses resulting from magnetic forces during operation of the device, mechanical stresses resulting from mechanically clamping or otherwise fixing the bulk amorphous metal magnetic components in place, or internal stresses caused by the thermal expansion and/or expansion due to magnetic saturation of the amorphous metal material.